Transistors are building blocks for integrated circuits. A transistor generally comprises a semiconductor substrate, a channel layer above or within the semiconductor substrate, a gate oxide layer and a gate stack above the channel layer, and source and drain diffusion regions at least one of on or in the surface of the semiconductor substrate. Electrical contacts are made to the gate stack and to the source and drain diffusion regions of the transistor. On top of the transistors, a plurality of layers of metal lines, formed with copper, aluminum, tungsten or cobalt, on different planes, are routed for signal transmission among transistors. The planes where the plurality of layers of metal lines reside are separated by inter-metal dielectric layers. Metal lines on different planes are connected through metal interconnect structures formed through the respective inter-metal dielectric layer. The metal interconnect structures may be generally formed with tungsten (W) or cobalt (Co).
In forming the metal interconnect structures, a chemical mechanical polishing (CMP) process is conducted to remove the additional metal film on top of the inter-metal dielectric layer. The metal CMP process utilizes a CMP slurry that includes a mechanical abrasion component, an oxidant(s) (or oxidizer(s)) and optional chemical metal etching component(s). The additional metal film is oxidized by the oxidant(s) so that the oxidized metal film can be removed by mechanical abrasion and/or chemical etching. As such, a metal CMP slurry includes one or more oxidizers to facilitate the metal oxidization.